Open Access Injection With Negative Wavelength Detuning...

Open Access

Injection With Negative Wavelength Detuningfor Multispectrum Frequency Generation andHopping Using SMFP-LDVolume 9, Number 5, October 2017

Bikash NakarmiHao ChenMuyong LeeYong Hyub WonShilong Pan, Senior Member, IEEE

DOI: 10.1109/JPHOT.2017.27302241943-0655 © 2017 IEEE

IEEE Photonics Journal Injection With Negative Wavelength Detuning

Injection With Negative WavelengthDetuning for Multispectrum Frequency

Generation and Hopping Using SMFP-LDBikash Nakarmi,1,2 Hao Chen,1 Muyong Lee,2 Yong Hyub Won,2

and Shilong Pan,1 Senior Member, IEEE

1Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, NanjingUniversity of Aeronautics and Astronautics, Nanjing 210016, China

2Convergence Optoelectronics and Device Engineering Laboratory, Department ofElectrical Engineering, Korea Advanced Institute of Science and Technology, Daejeon

431312, South Korea

DOI:10.1109/JPHOT.2017.27302241943-0655 C© 2017 IEEE. Translations and content mining are permitted for academic research only.

Personal use is also permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received June 28, 2017; revised July 14, 2017; accepted July 17, 2017. Date of publicationJuly 21, 2017; date of current version August 1, 2017. This work was supported in part by the NationalNatural Science Foundation of China under Grants 61650110515, 61422108, and 61527820, and inpart by the Fundamental Research Funds for the Central Universities. Corresponding author: BikashNakarmi (e-mail: [email protected]).

Abstract: We propose and demonstrate a novel method to generate multispectrum fre-quency and hopping from one frequency spectrum to another using a single external cavitybased Fabry–Perot laser diode (FP-LD) subjected to a single optical injection with negativewavelength detuning. The external cavity based FP-LD has a single dominant mode, single-mode FP-LD (SMFP-LD), and hence the beating of the dominant mode and the injectedbeam, and the beating of the injected beam and the corresponding side mode of SMFP-LDare possible with a single optical injection. The generated frequency can be tuned to a widerange of GHz to several THz. Individual or simultaneous RF signals generation and hoppingbetween microwave and millimeter wave spectrum are reported. The effect of the injectedbeam power on generating and hopping of RF frequency is analyzed in the case of negativewavelength detuning with the input beam injected on different modes of SMFP-LD. Also, thestability of beating wavelengths, power variations of beating wavelengths for an interval of1 hour, and linewidth of RF signals are observed.

Index Terms: SMFP-LD, negative wavelength detuning, frequency hopping, injectionlocking, microwave photonics.

1. IntroductionPhotonics generation of multi-spectrum frequency has drawn the extensive interest of researchersdue to its unsurmountable advantages such as the ability to generate tunable signals with higherfrequencies, simple configuration, transmission distance, and less susceptibility to electromagneticinterference. Due to these advantages, microwave photonics generation tremendously built its po-tentials for numerous applications such as wireless communication system [1], [2], radar system[3], biology [4], and others [5]–[7]. Microwave generation using optical fiber lasers, semiconductorlasers, and photonics crystal has been reported based on homodyne, heterodyne optical phase-locked loop [8], mode-locked lasers [9], optical frequency combs [10], side-band locking [11], andfeedback injection locking [12]. The optical heterodyning method of beating two separate tunablelasers at a photodiode is simple in configuration but optical sources should be coherent preserving

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widely adjustable frequency offset between them. Feedback injection locking has good spectral pu-rity but suffers from a limited tuning range attributed to loop bandwidth. Sideband injection lockingrequires reference source, hence, complex in configuration and expensive in cost. Also, reportedmethods are limited to the generation of single microwave frequency. Simultaneous microwavegeneration using quantum dot lasers with optical beating has been reported [13] which requirestwo external beams and a quantum laser diode. The requirement of two external lasers for simul-taneous generation of RF signals can be removed by using SMFP-LD and a single external beaminjection since SMFP-LD has a dominant mode under normal biasing condition. Also, the sameinjected single beam can be used for the generation of RF signals as well as a control signal forhopping/switching of generated RF signals of different bands. SMFP-LD has shown its significantpresence through demonstrating optical switches [14], wavelength converters [15], logic gates [16],latches [17], and others [18]–[20] due to its significant characteristics of self-injecting mode, highside mode suppression ratio, and less current requirement, outperforming other semiconductorlasers. Microwave generation using SMFP-LD with feedback injection locking and two color laserhas been demonstrated recently. However, for feedback injection locking it requires additional filtersand amplifiers for each RF output and two color laser diode lacks tunability of the two color modes[11], [21]. Instead of feedback injection locking, microwave can be generated from beating any ofthe side modes or the dominant mode and the injected beam, which are mode-locked in SMFP-LD.This will remove the necessity of individual filters and amplifiers required for each RF signal gen-eration using the feedback injection locking system. The RF signal generated by the beating of aninjection locked lasers provides better performance in noise and linewidth than free running laserbeams [22].

In this paper, we use injection locking with negative wavelength detuning in SMFP-LD for demon-strating our innovation on (1) generating millimeter wave, simultaneous microwave and millimeterwave, and microwave using SMFP-LD with a single external beam injection, (2) analyzing the effectof negative wavelength detuning, injected beam power, wavelength detuning in different side modesfor generating RF signals, and (3) multi-spectrum frequency hopping. The proposed scheme hasa wide tunable range from few GHz to THz, ranging from microwave to millimeter wave spectrum.The spectrum domain results from 5 GHz to 402 GHz with a signal to noise power ratio of morethan 25 dB is reported, which can be extended to THz [11]. The THz range is obtained when theexternal beam is injected to the higher side modes of SMFP-LD. The minimum power ratio obtainedbetween RF signal and noise is more than 24 dB and linewidth is measured below 250 KHz for42.5 GHz. The stability of beating wavelengths, the stability of power of beating wavelengths andthe linewidth of RF signal are measured. The proposed scheme provides a good tunable ability ofRF frequency, simple configuration, easy hopping from one RF signal to another (from microwaveto THz), and does not need any reference source unlike in other schemes [23], [24]. Hence, the pro-posed scheme establishes its potential in many applications such as multi-functional radar system,satellite networks, 5G mobile network, and radio over fiber applications.

2. Operating Principle and Experimental SetupThe fundamental principle behind the injection locking in SMFP-LD is the injection of the externalbeam to the right side of side modes of SMFP-LD, i.e., positive wavelength detuning (wavelengthdifference between the injected beam and the corresponding side mode) as shown in Fig. 1(a). InFig. 1(a), λa, λb, and λs3 indicate the wavelengths of the dominant mode, the injected beam, andthe 3rd side mode of the SMFP-LD, where an external beam is injected, respectively.

Two cases can be observed with positive injection locking: (1) the power of injected beam isnot sufficient to suppress the dominant mode of SMFP-LD and both dominant mode and injectedexternal beam exist known as weak injection locking as shown in Fig. 1(b) and (2) the power ofinjected beam is sufficient enough to suppress the dominant mode of SMFP-LD with sufficientamount of contrast ratio known as injection locking as shown in Fig. 1(c). With positive wavelengthdetuning, microwave generation is possible only by beating with a dominant mode and a weaklylocked injected beam. When the external beam is injected to any of the side modes of SMFP-LD,

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Fig. 1. Injection locking in SMFP-LD with positive wavelength detuning: (a) SMFP-LD under the normalbiasing condition and injected external beam, (b) weak injection locking, and (c) strong injection locking.

Fig. 2. Injection locking in SMFP-LD with negative wavelength detuning: (a) SMFP-LD under the normalbiasing condition and injected external beam, (b) millimeter wave, (c) simultaneous microwave andmillimeter wave, and (d) microwave.

only the generation of a millimeter wave is possible because of the beating of the dominant modeand the injected beam. With positive wavelength detuning, mode shifting of corresponding sidemode occurs and hence the microwave generation is not feasible in the side modes of the SMFP-LD, but the millimeter wave generation (due to the beating of the injected beam on side mode anddominant mode) is possible. Simultaneous microwave and millimeter wave generation with positivewavelength detuning between injected beam and corresponding side mode is not feasible due tomode shifting of corresponding side mode to the injected beam. In contrast, if the external beamis injected with negative wavelength detuning as shown in Fig. 2(a), following cases are observed.(1) The injected beam gains sufficient power without suppressing the dominant mode and withoutshifting the corresponding side mode to the injected beam as shown in Fig. 2(b). Hence, millimeterwave can be generated by beating the dominant mode and the injected beam, (2) on increasing thepower of the injected beam, the power of corresponding mode increases without suppressing thedominant mode, as a result, simultaneous generation of microwave and millimeter wave is feasibledue to beating of corresponding side mode and injected beam and by beating injected beamand dominant mode, respectively as shown in Fig. 2(c), and (3) on increasing power further, thedominant mode is suppressed, but still corresponding side mode does not shift to the wavelengthof the injected beam. Injected beam and corresponding side mode would still exist as shown inFig. 2(d), which provides microwave only.

The main difference between positive wavelength detuning and negative wavelength detuningis the mode shifting phenomena and the increase in mode power with an increase in the injectedbeam power. In positive wavelength detuning, the corresponding mode shifts to injected beam onincreasing the power of input beam and hence the third and fourth stage of negative wavelengthdetuning cannot be observed. We noted here, in negative wavelength detuning, the microwavegenerated by beating injected beam and corresponding side mode is possible only up to the half ofthe free spectral range (FSR) which is 0.58 nm (1.16 nm/2). Wavelength detuning greater than halfFSR will act as positive wavelength detuning to the previous mode. As a result, only one RF signalis possible by beating injected beam and the dominant mode.

The experimental setup of multi-spectrum frequency generation and hoping using SMFP-LDis shown in Fig. 3. The FP-LD used in the proposed scheme is specially designed and devel-oped by modifying commercial FP-LD which has multi-modes under normal biasing condition. The

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Fig. 3. Schematic of the experimental setup for generation and frequency hopping of the tunablemillimeter wave, millimeter and microwave and microwave using external cavity based Fabry-Perot laserdiode. PC: polarization controller, OSA: optical spectrum analyzer, ESA: electric spectrum analyzer.

developed SMFP-LD has a dominant self-locked single longitudinal mode with high side mode sup-pression ratio (SMSR). The SMFP-LD is obtained by eliminating the inclinations of 6° to 8° of thecoupling fiber present in conventional FP-LDs, thereby, forming an external cavity between the laserdiode and the fiber. The SMFP-LD consists of an FP-LD chip with a multi-quantum well of 300 μmand an external cavity length of 4 mm. By varying the temperature, a mode-matching condition isachieved for both cavities. The refractive index of the active region changes with the change in thetemperature. As a result, there is a change in the optical path length in the laser diode, providing theoptimal mode-matching condition for single-mode oscillation. This single-mode oscillation can betuned to another mode by varying the operating temperature, which gives the tunability of dominantmode wavelength to SMFP-LD. The self-locking mode of SMFP-LD is tunable over a wide range ofwavelengths with a wavelength difference of about 10 nm [25]. SMFP-LD shows similar characteris-tics to those of multimode Fabry-Perot laser diodes (MMFP-LDs) including the mechanical stability,wavelength stability, power stability of laser diode and also the output characteristics subjected tooptical injection of external beams. The only difference between SMFP-LD and MMFP-LD is thatthe former does not require an external probe beam for signal processing.

An LD driver (ILX Lightwave LDC-3724C) is used for biasing SMFP-LD, which has a facility ofchanging the biasing current and operating temperature. The operating temperature and the biasingcurrent define the wavelength and power of the dominant mode of the SMFP-LD. A tunable lasermodule which has features of varying wavelength and power can be tuned from 1530 to 1560 nm,and 5.5 to 16 dBm, respectively, is used for injecting external light beams to the SMFP-LD. Theexternal beam can be injected to any of the side modes in SMFP-LD including dominant modewith both positive and negative wavelength detuning. Attenuators, which are used at the outputof the laser source to minimize the power from the tunable laser source, are not shown in Fig. 3.Polarization controllers (PCs) are used to control the polarization state of the external beams tothe SMFP-LD which works in TE mode. The output from SMFP-LD is obtained through a circulatorand is divided into two branches by 50:50 coupler among which one is fed to an optical spectrumanalyzer (OSA, Yokogawa AQ6370C) set with 0.02 nm resolution, and another to an electricalspectrum analyzer (ESA, Agilent E4447A) set with 100 KHz resolution bandwidth via 50-GHzphotodiode (PD, U2t XPDV2120R).

3. Experimental Result and AnalysisAt first, we injected an external beam to the dominant mode to observe the generation of a microwavesignal of different frequencies that correspond to wavelength detuning between injected beam anddominant mode as shown in Fig. 4. We set the constant power of −5.2 dBm to the injected beam.In Fig. 4(a), the red line refers external injected beam that has wavelength detuning of −0.19 nm.Fig. 4(b) shows the result in optical spectrum domain with wavelength detuning of −0.19 nm. On

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Fig. 4. Basics of generating microwave signal (a) optical spectrum of SMFP-LD, (b) generationof microwave signal with an injection of external beam with negative wavelength detuning, and(c) ESA diagram for generated different frequencies of microwave corresponding to different wave-length detuning.

increasing the power of input injected beam, the injected beam gains power equivalent to dominantmode, as a result, RF signal with a frequency of 24.9 GHz is obtained by beating these two peaks.By varying the wavelength detuning with the dominant mode, RF signal with a different frequencycorresponding to that of wavelength detuning can be obtained, which is shown in Fig. 4(c). Weobserved the maximum RF frequency signal up to 42.5 GHz, the maximum frequency that can beobserved by ESA E4447A. However, the optical domain results are observed to the THz range.Fig. 4(c) shows electrical spectrum analyzer (ESA) result of different microwave frequency signals.The same phenomena can be obtained in any of the side modes of SMFP-LD for generatingmicrowave. The minimum power difference of signal to other frequency component is higher than26 dB in the experiment.

When an external beam is injected into any of the side modes of SMFP-LD, the dominantmode and the injected beam are used as beating wavelengths to generate the millimeter wavewhereas, external injected beam and corresponding side mode are required beams for generatingmicrowave. In order to show a tunable feature of SMFP-LD, it is again biased by 23 mA biasingcurrent at operating temperature of 21 °C. With these operating conditions, the dominant modeof SMFP-LD is shifted to 1540.74 nm with SMSR of 36 dB. In order to generate both microwaveand millimeter wave simultaneously, the injected beam should be injected to side modes ratherthan the dominant mode. For the demonstration of our proposed idea of multi-spectrum frequencygeneration and hopping, we injected external beam on 3rd mode with wavelength detuning of−0.178 nm. The external beam can be injected to any of the modes with any wavelength detuning,the only parameter to control for different mode and different wavelength detuning is the power ofthe injected beam. With sufficient power of the external beam, the injected beam to 3rd mode gainsenough power and hence beating of dominant mode, 1540.74 nm, and injected beam on 3rd mode,1543.93 nm, occurs which gives the millimeter wave of 402.3 GHz, corresponding to a wavelengthdifference of 3.19 nm as shown in Fig. 5(a).

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Fig. 5. Spectra of generated RF signals. Optical spectrum: (a) millimeter wave, (b) simultaneous mi-crowave and millimeter wave, and (c) microwave. (d) Electrical domain microwave.

Since the external beam is injected with negative wavelength detuning, increasing the powerof the external beam contributes not only to increase the power of the injected beam, but alsohelps to obtain sufficient amount of power by corresponding side mode without suppressing thedominant mode as shown in Fig. 5(b). As a result, simultaneous microwave and millimeter wavesare generated. It is possible because of the negative wavelength detuning which prevents thewavelength shifting of the side mode of the SMFP-LD to the injected beam unlike in positivewavelength detuning. With negative wavelength detuning, it is observed that the decrease in powerof dominant mode is not significant as compared to power gained by side mode. Hence, beating ofside mode and injected beam as well as the beating of injected beam and dominant mode of SMFP-LD is possible, which makes the feasible generation of both microwave of 22.3 GHz (corresponds tothe wavelength detuning of −0.178 nm) and millimeter wave of 402.3 GHz simultaneously. Besidesthe millimeter wave of 402.3 GHz, an additional RF signal of 422.6 GHz (402.3 GHz + 22.3 GHz)can also be obtained through the optical beating of the dominant mode of the SMFP-LD and the3rd side mode as shown in Fig. 5(b). The dominant mode will be suppressed with further increasein power of external beam and generates the microwave of 22.3 GHz only, as illustrated in Fig. 5(c).

In Fig. 5(b), harmonics are seen on the both sides of the dominant mode and the 3rd modewhereas in Fig. 5(c), harmonics are only seen on the 3rd mode which is due to the four-wavemixing. Since the harmonics have less power compared to the dominant mode, injected beam andcorresponding side modes, the effect of harmonics is negligible in the generation of the RF signal.The power of harmonics generated is maintained at least 26 dB below the power of injected beam,dominant mode, and corresponding side mode while generating simultaneous microwave andmillimeter wave. The wavelength difference between generated harmonics is twice the wavelengthdetuning, 0.35 nm. The electric domain result of the generated microwave of 22.3 GHz is shown inFig. 5(d) which has a signal to noise power ratio of 26.8 dB. The millimeter waves of 402.3 GHz and422.6 GHz are not observed in electrical spectrum analyzer (ESA) due to a limited operating rangeof the photodiode (PD) which has a bandwidth of 45 GHz and ESA E4447A that has a maximumrange of 42.5 GHz. The frequency of the generated signal can be varied by varying the wavelengthdetuning and injecting beam from one side mode to another side mode, which provides the tunablefeature of the proposed multi-spectrum generator.

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Fig. 6. Effect of negative wavelength detuning on the generation of microwave and millimeter wave withnegative wavelength detuning of (a) 0.08 nm, (b) 0.12 nm, and (c) 0.17 nm.

The generation of the microwave with the corresponding mode has the tunability range of about0.58 nm. Higher than 0.58 nm in the negative wavelength detuning, the wavelength detuningact as positive wavelength detuning to the next lower side mode of the SMFP-LD. In this case,simultaneous microwave and millimeter wave generation is not possible because correspondingside mode cannot gain sufficient power required for an optical beating. However, the generationof only one RF signal, either microwave or millimeter wave, that corresponds to the wavelengthdetuning of higher than 0.58 nm is possible with the optical beating of the dominant mode and theweakly locked injected beam.

Fig. 6 illustrates the input power required for generating multi-spectrum frequencies and theimpact of input injected beam on dominant mode, corresponding side mode, and injected modewhen the external beam is injected on the 3rd side mode of SMFP-LD with a wavelength detuningof −0.08 nm, −0.12 nm, and −0.17 nm. It is clear from Fig. 6(a) that three different regions can beattained with negative injection locking in contrast to positive injection locking where only two stagesknown as weak injection locking without suppression of the dominant mode and injection lockingwith fully suppressed dominant mode are observed. Hence, only a single RF signal is feasiblewithout negative wavelength detuning. In Fig. 6(a), the injected beam gains sufficient amount ofpower when the input injected beam power is higher than −16 dBm. As a result, millimeter wavecan be obtained by beating the dominant mode and the injected mode whose wavelength differenceis 3.19 nm.

Beyond −13.5 dBm, corresponding side mode gains the power on increasing the power of theinjected beam. As a result, three beams exist with sufficient power for optical beating, as a result,millimeter wave corresponding to 3.19 nm and microwave corresponding to the wavelength detuningof 0.08 nm is generated. This simultaneous generation of microwave and millimeter wave is possibledue to the injection locking with negative wavelength detuning. A further increase in the power ofthe injected beam, the dominant mode of SMFP-LD is suppressed with ON/OFF contrast ratio ofmore than 26 dB, as a result, only microwave is generated as shown in Fig. 6(a). Similarly, for thewavelength detuning of −0.12 nm, the power requirement for generating different RF frequenciesand hoping of generating RF frequencies is illustrated in Fig. 6(b). Millimeter wave is generated

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Fig. 7. Generation of multi-spectrum frequency; millimeter wave, simultaneous microwave and millimeterwave and microwave with a constant optical input injected power of −5.2 dB for (a) 1st mode, (b) 3rdmode, and (c) 5th mode.

when the power of injected beam is below −10 dBm. By increasing the injected power from−10 dBm to −8.5 dBm, simultaneous microwave and millimeter wave are obtained. When the inputpower is increased beyond −8.5 dBm, the dominant mode is suppressed with sufficient amount ofpower, which is recorded as 23 dB, and hence only microwave is generated. Fig. 6(c) shows thethreshold power increases to −7 dBm for generating higher frequency microwave corresponding tothe wavelength detuning of −0.17 nm. With these results, we can conclude that irrespective of thewavelength detuning, three regions (a) millimeter wave, (b) simultaneous microwave and millimeterwave and (c) microwave can be obtained by only changing the power of the injected beam while allother variables are kept constant. Also from Fig. 6, it is clear that the dynamic input power range forgenerating the microwave corresponding to the higher wavelength detuning is larger compared tothat of lesser wavelength detuning. The input dynamic-power-range of 1.5 dB and 2 dB is obtainedfor the case of −0.12 and −0.17 nm wavelength detuning, respectively.

The feasibility of generating millimeter wave, simultaneous microwave and millimeter wave, andmicrowave by injecting the input beam to any of the side modes is shown in Fig. 7. For the verifica-tion of the generation of millimeter wave, simultaneous generation of both microwave and millimeterwave, and microwave in any of the modes of SMFP-LD, TLS is further injected to 1st mode and5th mode of the SMFP-LD with a constant injected beam power of −5.2 dBm. Injecting beam infarther modes gives high-frequency millimeter wave signal, ranging to few THz. In order to showthe possibility of different wavelength detuning that can be used for generating multi-spectrum fre-quencies, we varied wavelength detuning by changing the wavelength of injected external beam.The effect of changing wavelength detuning on the power of dominant mode, injected mode, andside mode with a constant power of an external injected beam is shown in Fig. 7. Fig. 7(a)–(c)show the observed power of the output beams when an external beam is injected to 1st, 3rd, and5th mode of the SMFP-LD. In Fig. 7(a), both dominant mode and injected external beam is logichigh for the wavelength detuning range from −0.19 nm to −0.28 nm and 0.1 nm to 0.25 nm withthe maximum power difference (between the dominant mode and the injected beam) of 3 dB and5 dB, respectively, whereas the side mode has a power gain of about 2 dB only, with the injection ofexternal beam. Hence, in these regions, only millimeter spectrum signal with different frequencies

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Fig. 8. Output signal performance measurements (a) power stability of beating wavelength and (b)linewidth of generated microwave signals.

are possible with the constant input injected beam power of −5.2 dBm. On the contrary, wavelengthdetuning of −0.18 nm with corresponding side mode, a microwave of 22 GHz is generated alongwith millimeter wave corresponding to 142 GHz. On increasing wavelength detuning from −0.18 nmto −0.07 nm, both side mode and injected mode are high which generates microwave spectrumsignals with different frequencies corresponding to wavelength detuning. Millimeter wave is notgenerated because the dominant mode is suppressed. Beyond −0.07 nm to 0.1 nm, the dominantmode is suppressed together with the wavelength shifting of side mode of the SMFP-LD to theinjected beam due to injection locking phenomena and hence, no any microwave and millimeterwave is generated between the detuning range of −0.07 nm to 0.1 nm. However, the uncovereddetuning range of −0.07 nm to 0.1 nm can be changed by varying input injected beam power andhence the wavelength detuning range of −0.07 nm to 0.1 nm can be used for the generation ofRF signal. The similar phenomena are observed in 3rd mode and 5th mode, which shows thereexists a wavelength detuning range where both microwave and millimeter wave can be generatedwith constant input injected power. The small difference in ON/OFF contrast ratio and other ob-servation is due to the variation in the power of the side mode under normal biasing condition.By changing the mode where the input beam is injected, the frequency of the millimeter wave canbe changed. Fig. 7 also shows that injecting input beam on higher side modes will provide bettersimultaneous microwave and millimeter wave generation since the power difference of all threebeams are small at the specific frequency of microwave and millimeter. Also, the same power levelof three beams can be obtained by injecting an external beam of a lower power to the nearer sidemode.

The power stability of beating wavelengths and linewidth of generated microwave signals isillustrated in Fig. 8. We observe the variation of beating wavelengths over a period of 1 hour withan interval of 5 min. The difference between beating wavelengths is constant within the resolutionof OSA of 0.02 nm. This is an important property because the wavelength variation is the mostinfluencing parameter for the RF signal generation using an optical beating. Fig. 8(a) shows thepower variation of the beating wavelengths (dominant mode, injected beam, and corresponding sidemode) while generating simultaneous microwave and millimeter wave RF signals. The maximumpower variation of 0.2 dB is observed during the observation period of 1-hour time. The linewidthfor microwave of 6 GHz to 42.5 GHz with a constant power of the input injected beam is illustratedin Fig. 8(b). The linewidth increases from about 100 KHz to 230 KHz for 42.5 GHz. The increasein linewidth with an increase in frequency of the generated microwave is due to the constantinjection power used for the generation of all frequencies. Generating higher frequency requireshigher wavelength detuning and the farther modes from the dominant mode of SMFP-LD. Hencethe strength of injection decreases with higher wavelength detuning and injecting to a farthermode with constant power that contributes on linewidth of higher frequency generation, whichcan be reduced by changing the power of input injected beam. With the increase in the injectionratio by increasing the input beam power, linewidth can be reduced further for respective RFsignals [26].

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4. ConclusionWe proposed and experimentally demonstrated multi-spectrum simultaneous photonics signal gen-eration and hoping that ranges from 6.7 GHz to 402 GHz. The range of generating an RF signalcan be further increased to the THz range by injecting beam on farther side modes of the SMFP-LDsubject to a single external beam injection using negative wavelength detuning. Increasing thepower of the injected beam with a negative wavelength detuning, we observe mainly three states:(a) increase on the power of the injected beam without suppressing the dominant mode, hence, thedominant mode and the injected beam are beating wavelengths for the generation of RF signal, (b)increase the power of corresponding side mode, as a result, two simultaneous RF signals can beobtained with different frequencies and (c) suppress the dominant mode but the injected beam andthe corresponding side mode exist, which are the beating wavelengths for the RF signal generation.These three states can be obtained by controlling the power of injected beam and thus hoppingfrom millimeter to simultaneous microwave and millimeter wave, to microwave is attained. The effectof the power of the injected beam is analyzed to different modes of SMFP-LD, which shows thesame result of the transition to the generation of RF signals. The generated microwave has signalto noise power ratio of more than 26 dB. The stability and power variation of beating wavelengthsare observed for about 1-hour time interval which shows the variation in wavelengths are below theinstrument resolution of 0.02 nm and the power variation is about 0.2 dB. The maximum linewidthof less than 250 KHz is measured for 42.5 GHz RF signal. With these observations and results, theproposed scheme can be used for optical wireless network, spectroscopy and Radar system. Also,by applying a certain pattern for varying the power of the injected beam, secure data transmissioncan be attained as the carrier RF frequency changes with changes in the injected power.

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[22] Y. S. Juan and F. Y. Lin, “Photonic generation of broadly tunable microwave signals utilizing a dual-beam opticallyinjected semiconductor laser,” IEEE Photon. J., vol. 3, no. 4, pp. 644–650, Aug. 2011.

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[24] S. X. Chew et al., “Distributed optical signal processing for microwave photonics subsystems,” Opt. Exp., vol. 24, no. 5,pp. 4730–4739, 1992.

[25] Y. D. Jeong, Y. H. Won, S. O. Choi, and Y. Yoon, “Tunable single-mode Fabry-Perot laser diode using a built-in externalcavity and its modulation characteristics,” Opt. Lett., vol. 31, no. 17, pp. 2586–2587, 2006.

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